1. Field of the Invention
This invention relates to apparatus for casting metal strip, particularly strip of metallic glass alloys.
2. Description of the Prior Art
For purposes of the present invention, a strip is a slender body whose transverse dimensions are much less than its length, including wire, ribbon, filament and sheet, of regular or irregular cross section.
Methods for making metal strip directly from the molten metal are known. These may involve, for example, jetting molten metal through an orifice and cooling the metal, either in free flight or by contact with a chill body, to obtain continuous strip. Typically, the molten metal is pressurized in a crucible, which has at its bottom a casting nozzle having an orifice in the shape of the desired cross section of the metal jet. Usually, the orifice of the casting nozzle is small, on the order of about 0.2 mm to about 1.0 mm diameter, and plugs easily in operation. Unplugging is not simple, since a body of molten metal is maintained above the orifice. Another method for making metal strip directly from the melt is the "planar flow casting" (PFC) method disclosed in U.S. Pat. No. 4,142,571, issued Mar. 6, 1979, to Narasimhan. This patent discloses apparatus which comprises a movable chill body, a slotted nozzle in communication with a reservoir for holding molten metal and means for effecting expulsion of the molten metal from the reservoir through the nozzle and onto the moving chill surface. The slotted nozzle is located in close proximity to the chill surface. The nozzle of the PFC method not only has the plugging problems indicated above, but it must also be precisely oriented relative to the chill surface.
In casting processes of the types mentioned, as well as in other metal casting processes, it is convenient to be able to replace the casting nozzle while maintaining a charge of molten metal in the crucible. The advantages of an externally replaceable nozzle were recognized in U.S. Pat. No. 4,154,380, issued May 15, 1979, to R. W. Smith. That patent discloses a casting nozzle comprising three elements, including a nozzle body, a replaceable orifice member and retaining means for holding the orifice member in sealing engagement with the nozzle body. A simpler design for an externally replaceable nozzle is desirable.
U.S. Pat. No. 3,679,105, issued July 25, 1972, to Ratcliffe, discloses a nozzle arrangement for a molten metal container vessel that allows installation of the nozzle from outside the vessel. However, that arrangement is not adapted for replacing the nozzle while the vessel contains molten metal.
In both the Smith and the Ratcliffe designs, the nozzle is rigidly secured to the molten metal container vessel by a screw arrangment. Since the vessel must be of a refractory material, not easily machined, the need to drill and tap into the vessel is an undesirable feature. In addition, thermal expansion can cause a change in the orientation of the nozzle during heating. If the materials of the nozzle and vessel are different, thermal expansion could also cause large stresses and consequent failure of the nozzle, the vessel or both.